The invention relates to a method of decontaminating a reactor coolant system of a pressurized water reactor (PWR) and, more particularly, to a method of decontaminating the primary loop of a reactor coolant system without decontaminating the reactor vessel.
Reactor coolant systems of pressurized water nuclear reactors form oxide corrosion films on the metal surfaces wetted by the coolant as the reactors operate and produce power. The coolant is essentially demineralized water containing parts per million additions of boron and lithium and small amounts of dissolved hydrogen. Commonly employed metals in reactor coolant systems include stainless steels and nickel base alloys and the corrosion films generally include iron, nickel and chromium-containing oxides. In the course of operation, corrosion particles (generally referred to as "crud") are eroded from the films by high velocity coolant and are transported to the cores of the reactor vessels where they become activated by the fuel assemblies contained therein. The activated crud subsequently released from the cores and transported by the high velocity coolant from the reactor vessels throughout the balance of the systems then redeposit in the corrosion films on the metallic surfaces of the primary loops associated with the reactor vessels. A "primary loop" of a reactor coolant system includes a steam generator, a reactor coolant pump and a hot leg and a cold leg interconnecting the reactor vessel with the steam generator.
Redeposited crud raises the background radiation levels of the plants to levels which drastically limits the permissible exposure time of personnel in the area around the reactor coolant systems during plant outages and other maintenance activities. Various chemical processes for reducing the background radiation levels by removing radioactive corrosion films and adjacent metal from the primary coolant metal surfaces have been proposed. See, e.g., M. E. Pick et al., "Chemical Decontamination Of Water Reactors--ECGB Developments And The International Scene", Nuclear Energy, Vol. 22, No. 6, 1983, pages 433-444, for a summary discussion of several permanganate, LOMI, CAN-DECON and other dilute chemical processes.
A considerable portion of the activated crud is redeposited in the steam generator channel heads and in the first two or three feet of the steam generator tubes extending above the channel heads. Relatively little crud deposits higher than about three feet in the steam generator tubes. In addition, much of the time spent by personnel in the course of plant outages is in the vicinity of the channel heads. Accordingly, the channel heads have been decontaminated in the preliminary steps of several outages. In one reported decontamination, the channel head was isolated from the reactor vessel, coolant pumps and interconnecting hot leg and cold leg piping by nozzle dams located in the channel head. Water diluted with small amounts of decontaminating chemicals then was circulated between the hot leg and the cold leg sides of the channel head and a decontamination treatment unit. See, in this regard, R. M. Orsulak et al., "Millstone II Decontamination", Water Chemistry Of Nuclear Reactor Systems 3, British Nuclear Energy Society, 1984. While the radiation levels of the channel heads have been reduced by such methods, the background radiation levels associated with the coolant pumps and the hot leg, cold leg and appurtenant piping have not been reduced.
It has been proposed to decontaminate entire reactor coolant systems using dilute chemical decontamination processes. See, e.g., U.S. Pat. No. 5,089,216 to Schlonski et al. which proposes to decontaminate entire systems using LOMI or CAN-DEREM dilute chemical processes. In practice, these processes will be applied in successive steps together with preliminary alkaline permanganate addition steps to first oxidize the chromium in the oxide films. However, there are cost and technical questions regarding entire system decontaminations. For example, the industry has questioned the advisability of decontaminating a reactor vessel (and perhaps the fuel assemblies in the core) in entire system decontaminations. Also, the industry has questioned the advisability of exposing the thin tubes in the steam generators to the corrosive decontamination chemicals employed in these dilute chemical decontamination processes.